1. Field of the Invention
The invention relates to an insulated wire used for a coil of electrical equipments such as rotating equipment or electric transformer, in particular, to an insulated wire in which an insulating covering layer is formed by an insulating coating made of polyamide-imide resin insulating material.
2. Description of the Related Art
In general, an insulated wire (enamel wire) provided with an insulating covering layer composed of one or two or more insulating coatings obtained by applying and baking an insulating material, in which resin such as polyimide, polyamide-imide or polyester-imide is dissolved in an organic solvent, on a periphery of a metal conductor (a conductor) having a cross-sectional shape consistent with or conforming to an intended use or shape of the coil (e.g., having a circular or rectangular shape) is broadly used for a coil of electrical equipments such as rotating equipment or electric transformer.
Inverter control has been used to drive the electrical equipments such as rotating equipment or electric transformer, and in such electrical equipments using the inverter control, when inverter surge voltage generated by the inverter control is high, the generated inverter surge voltage could enter the electrical equipments. When the inverter surge voltage enters the electrical equipments, partial discharge may occur in an insulated wire composing the coil of the electrical equipments due to the inverter surge voltage, which may lead to deterioration of or damage to the insulating coating.
A method of preventing such deterioration of the insulating coating caused by the inverter surge voltage (deterioration of the insulating coating caused by the partial discharge) is known in which a lifetime of the insulating coating against the partial discharge is improved (surge resistance is improved) by using an insulated wire having an insulating coating formed by applying and baking an insulating material, which is obtained by dispersing, e.g., organosilica sol in resin solution made of polyamide-imide resin, etc., (a partial-discharge-resistant insulating material), on a conductor to prevent the insulating coating from deteriorating or being damaged even though the partial discharge occurs (e.g., see JP-A-2006-302835 and JP-B-3496636).
In addition, another method of preventing deterioration of the insulating coating caused by the inverter surge voltage is known in which an insulated wire is provided with an insulating covering layer having, e.g., an insulating coating formed on a conductor and an extruded covering layer formed thereon and made of polyphenylene sulfide (PPS) in order to increase partial discharge inception voltage, thereby preventing the occurrence of the partial discharge in the insulated wire (e.g., see JP-B-4177295).
In recent years, a hybrid vehicle, etc., has been becoming popular in the context of energy conservation, etc., and the electrical equipment used for such application is controlled by an inverter at voltage higher than a conventional art since a compact size and high voltage driving are desired in order to improve fuel consumption and engine performance of the hybrid vehicle, etc.
In addition, in recent years, it is further required to improve a space rate of an insulated wire to a motor for further downsizing and high-efficiency of the electrical equipment controlled by the inverter, thus, the partial discharge is becoming more likely to occur in the insulated wire than in the conventional art.
Therefore, the latest insulated wire is required to have partial discharge inception voltage higher than the conventional art (e.g., partial discharge inception voltage of 970V or more) in order to prevent the occurrence of the partial discharge, however, an insulated wire described in JP-A-2006-302835 and JP-B-3496636, in which an insulating covering layer provided on a conductor is composed of only an insulating coating formed by applying and baking an insulating material, does not have the high partial discharge inception voltage as described above, hence, resistance against the partial discharge may be insufficient.
On the other hand, in JP-B-4177295, although it is possible to provide the partial discharge inception voltage higher than the conventional art as described above by providing an extruded covering layer made of PPS, etc., on the insulating coating, it is necessary to provide an insulating coating in order to impart adhesion between the conductor and the extruded covering layer. Therefore, when the insulating coating and the extruded covering layer are formed, different fabrication processes, at least a painting process for forming an insulating coating and an extrusion process for forming an extruded covering layer, are required. Thus, since it takes time and effort because of multiple fabrication processes and it is necessary to form the extruded covering layer having a different property on the insulating coating, there is a problem that complicated operations and conditions, etc., are required for fabrication, which accordingly leads to an increase in cost.